>>> client = MongoClient(directConnection=True)
>>> client.topology_description
<TopologyDescription id: 62bb5c0eca0eb781b7d532a8, topology_type: Single, servers: [<ServerDescription ('localhost', 27017) server_type: RSPrimary, rtt: 0.0005203270000002647>]>
>>> with client.start_session() as s, s.start_transaction():client.t.t.count_documents({}, session=s)
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/shane/git/mongo-python-driver/pymongo/collection.py", line 1843, in count_documents
    return self._retryable_non_cursor_read(_cmd, session)
  File "/Users/shane/git/mongo-python-driver/pymongo/collection.py", line 1849, in _retryable_non_cursor_read
    return client._retryable_read(func, self._read_preference_for(s), s)
  File "/Users/shane/git/mongo-python-driver/pymongo/mongo_client.py", line 1405, in _retryable_read
    return func(session, server, sock_info, read_pref)
  File "/Users/shane/git/mongo-python-driver/pymongo/collection.py", line 1838, in _cmd
    result = self._aggregate_one_result(sock_info, read_preference, cmd, collation, session)
  File "/Users/shane/git/mongo-python-driver/pymongo/collection.py", line 1712, in _aggregate_one_result
    session=session,
  File "/Users/shane/git/mongo-python-driver/pymongo/collection.py", line 301, in _command
    user_fields=user_fields,
  File "/Users/shane/git/mongo-python-driver/pymongo/pool.py", line 758, in command
    session._apply_to(spec, retryable_write, read_preference, self)
  File "/Users/shane/git/mongo-python-driver/pymongo/client_session.py", line 974, in _apply_to
    "%r" % (read_preference,)
pymongo.errors.InvalidOperation: read preference in a transaction must be primary, not: PrimaryPreferred(tag_sets=None, max_staleness=-1, hedge=None)